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I'm going to buy the book because I'm going to America.
-> You buy the book before you go, and you buy it because you are going.

アメリカに行ってから本を買う

I'm going to buy the book after I go to America.
-> You buy the book after you go. The reason you buy the book may not be related to you going there at all.

アメリカに行ったから本を買う

I'm going to buy the book because I went to America.
-> You buy the book after you went, and you buy it because you went.

Note: in 行ってから本を買う the location where you buy the book can be in America or after you return back to your place. You may need to use specify あそこで to show that you are going to buy the book in America, or use 行って来て to show that you are going to buy it after you return home.

so if i had 海外に行くから本を買う and 海外に行ってから本を買う, the difference would be buying the book before i go abroad for the first one and buying the book after i have arrived in the foreign country?
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Mark HosangJun 2 '11 at 4:39

@Mark Yes. You got the difference right :) Though in your first example, it is for the very reason of going abroad that you are going to buy the book (some travelers' guide book or something).
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LukmanJun 2 '11 at 4:46

and 行ってから would be i am in Japan, saying i will buy a book once i am in america and 行ったから would be, because i went to america i bought a book (though don't know if i bought it in america or japan) sorry, this has been something i've struggled with a very long time.
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Mark HosangJun 2 '11 at 4:56

@Mark, as it is 買う, it means you will buy it because and after going to and getting back from America. If you were still in America you'd say 来たから。
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jbcreixJun 27 '11 at 15:09